SAW components use acoustic waves which travel at the speed of sound. The SAW components are preferred over widely used transmission line components because acoustic waves have a substantially shorter wave length at operating frequency than electromagnetic waves which travel at the speed of light. Therefore, for a given operating frequency, a SAW resonator filter provides a smaller sized structure than a transmission line structure, therefore, making them suitable for miniaturized radio frequency applications. Furthermore, SAW structures are easily integratable with other active circuits, such as amplifiers and mixers, which are produced using conventional integrated circuit technologies. For the above reasons, the popularity of SAW structures in radio frequency applications, especially in resonator filter applications, has been steadily increasing.
FIG. 1 depicts the diagram of a conventional single track SAW filter structure 100 comprising SAW transducers 110 and 120 and reflectors 130, 140 and 150 disposed on a piezoelectric substrate 105. The filter 100 is characterized by two resonators 180 and 190 which are positioned on a single track (shown on FIG. 1 as being separated by dotted line) and are resonant at a resonator frequency. As illustrated, the transducer 110 is an input transducer being coupled to a source 195 having a source impedance R.sub.S and the transducer 120 is an output transducer which is coupled to a load R.sub.L. Resonator 180 includes the transducer 110 and the reflectors 130, and the resonator 190 includes the transducer 120 and reflectors 150. The input and the output transducers 110 and 120 each include a plurality of interdigitated open-ended fingers 114 having a center-to-center spacing equal to 1/2 the wave length at the resonator frequency. In the filter 100, the reflector 140 is shared by the resonators 180 and 190 and, by leakage, provides the in-band coupling between them.
However, the SAW filter 100 suffers from poor selectivity and out-of-band response. This is because the reflectors 130, 140, and 150 have reflectivity in only a small band around a center frequency and outside of this band the acoustic waves propagate freely due to lack of reflectivity. The lack of out-of-band reflectivity causes direct coupling between the input transducer 110 and the output transducer 120, thus degrading the overall frequency response of the SAW filter.
In order to resolve the out-of-band response problem of the single track filter arrangement, one prior art approach utilizes a dual track filter arrangement. Referring to FIG. 2, one such dual track SAW filter 200 is shown. The filter 200 includes a first resonator 210 and a second resonator 220 which are symmetrically positioned along a common conductive track 230. This arrangement relies on longitudinal coupling provided by the common track 230 for achieving acoustic coupling between the first and the second resonators 210 and 220. In this dual track arrangement, direct out-of-band coupling encountered with the SAW filter 100 is avoided. However, since the acoustic waves propagating within the resonators 210 and 220 travel along two substantially separate and parallel tracks, the acoustic coupling provided by this arrangement is very small and unpredictable. Thus, filter design necessarily leads to a trial and error process, the result of which does not satisfy applications where large acoustic coupling are needed.
Therefore, it is desired to design a SAW filter with a significantly improved out-of-band response which avoids the drawbacks encountered in prior art approaches.